The present invention relates to a mobile radio communication system (hereinafter called xe2x80x9cCDMA mobile communication systemxe2x80x9d) using code segmenting multiplex connection technology, and to a technique for performing more optimum handover.
In a mobile communication system, a mobile station selects or performs switching to a base station used as a communicating partner according to its movement, whereby the mobile station is able to communicate therewith continuously even if it moves. This base station switching operation is called xe2x80x9chandoverxe2x80x9d. A base station controller selects a base station low in pass loss of an electromagnetic wave transferred between a mobile station and each base station and large in signal to interference power ratio (hereinafter called xe2x80x9cSIRxe2x80x9d) and connects the selected base station and the mobile station. In the present application, this operation is called xe2x80x9chandover adding operationxe2x80x9d. On the other hand, the base station controller releases or sets free a connection to a base station related to handover, which is large in pass loss and low in SIR in order to reduce the occupation of an unnecessary radio line. In the present application, this operation is called xe2x80x9chandover deleting operationxe2x80x9d. Thus, the mobile station is connected to a plurality of base stations which resembles in pass loss and SIR and sets free the connection to unnecessary base stations, whereby a high-quality communication state thereof is kept with the required minimum radio resources. Incidentally, the inventions disclosed in Japanese Patent Application Publication Laid-Open Nos. Hei 5-316023 and Hei 10-200946 are known as prior arts.
On the other hand, when a mobile station moves in building-dense regions such as cities, a pass loss and SIR suddenly change. Since only line quality of a perch channel at the present time makes an opportunity to perform handover in the related art, a handover adding operation and a handover deleting operation are frequently carried out when the pass loss and SIR continuously change suddenly.
The related art is based on the premise that a mobile station is connected to one base station alone, and never communicates with a plurality of base stations simultaneously. On the other hand, a CDMA mobile station is capable of being connected to a plurality of base stations simultaneously by taking advantage of a CDMA system (it is not uncommon that the mobile station is connected to three base stations simultaneously). Therefore, it is difficult to suitably select a base station brought to a handover additional candidate or a base station brought to a handover deletion candidate. Thus, an unsuitable candidate might be selected. Since the handover is processed using a connected time or the mean value of maximum transmitter power levels in the related art, a base station which is free of existence in a moving direction, i.e., is not to be originally connected, is brought to a handover candidate depending on a call time or the like of a mobile station having carried out the handover in the past. Also, even in the case of a sector type base station in which one cell is divided into a plurality of areas, handover processing much than required might be performed. There is, for example, a case in which a mobile station approaches right at the sector base station and radio waves from a plurality of sectors can be received with substantially the same pass loss and SIR.
If the base station not to be connected or originally set free is taken as the candidate for handover processing in this way, then the repetition of unnecessary handover adding/deleting operations might occur continuously. Thus, if the unnecessary handover adding operation and handover deleting operation are repeated, various resources such as CPU, a radio resource, etc. are consumed than required by the mobile station, base stations and base station controller. This consumption would bring about a delay with respect to the operation to be originally performed by the corresponding mobile station or another mobile station, thus leading to degradation in communication state. Further, the unsuitable execution of handover results in cutting-off of a calling.
Therefore, an object of the invention of the present application is to effectively take advantage of resources by reducing unnecessary handover. Another object of the invention of the present application is to reduce unnecessary handover so as to improve a processing delay of a system and provide more stable and high-quality communications.
According to the invention of the present application, as a method of solving the above-described problem, time-sequential data about pass losses and SIR (or transmitter power level and communication quality at base stations) of channels intended for the measurement of received levels, which are transmitted from connected base stations and non-connected base stations to a mobile station, and learning data are compared, and a handover additional base station or a handover deletion base station is selected from the degree of similarity thereof. As the learning data, ones (hereinafter called xe2x80x9creference tablesxe2x80x9d) obtained by updating the above-described time-sequential data, based on the result of the past handover adding operation and handover deleting operation are used. A handover candidate base station is selected based on time-sequential data about pass losses and SIR (or transmitter power level and communication quality at base stations) from a plurality of base stations, i.e., the transition of their change. Thus, since the suitable base station can be selected, it is possible to restrain the repetition of the handover operation, control the use of unnecessary radio resources in its turn and suppress the influence of communication quality to the corresponding mobile station and other mobile stations. The use of information from connected base stations and non-connected base stations makes it possible to provide satisfactory handover even in the case of a CDMA mobile communication system connected to a plurality of base stations simultaneously.
Further, handover operating conditions are determined based on combinations of time-sequential data such as the aforementioned pass losses and SIR or pass losses and transmitter power levels, or pass losses and frame error rate (hereinafter called xe2x80x9cFERxe2x80x9d), etc. so that connections much than required are avoided when the state of communications with each connected base station is good. When the state of the communications with each connected base station is poor in reverse, a handover adding operation is executed earlier rather than when the state of the communications therewith is good, so that the corresponding mobile station can be connected to a plurality of base stations.